Lipoprotein lipase, a glycoprotein, acts on chylomicrons at the luminal surface of capillaries. Tissue cells other than endothelium are believed to synthesize the enzyme. Distribution of lipoprotein lipase in heart of fed and fasted mice was studied with an indirect immunocytochemical method using chicken antiserum to bovine milk lipoprotein lipase, which cross-reacts with mouse lipoprotein lipase, and gold- or ferritin-labeled antibodies. This approach demonstrates protein which could be either an active or an inactive form of the enzyme. Lipoprotein which could be either an active or an inactive form of the anzyme. Lipoprotein lipase was found in capillaries and cardiac myocytes. The gold labeling of the capillary surface located lipoprotein lipase at sites of activity in capillaries. The gold labeling of cardiac myocytes in organelles involved in the synthesis, of glycoproteins indicates that cardiac myocytes are a source of lipoprotein lipase. Fasting increased the amount of lipoprotein lipase, demonstrated immunocytochemically, in myocytes, extracellular space and capillaries. Brown adipose tissue of mice with Combined Lipase Deficiency (cld/cld) have very low levels of lipoprotein lipase activity. Yet 4 X normal amounts of lipoprotein lipase protein. Cultured cells from brown adipose tissue of cld/cld mice were shown by immunofluorescence to contain intracellular lipoprotein lipase whereas cells from normal littermates contained none. These findings suggest that cell cultures of brown adipose tissue can be used to study synthesis and secretion of lipoprotein lipase.